(a) Field of the Invention
The present invention relates to a method for fabricating a semiconductor device and, in particular, to a method for fabricating semiconductor devices having gate oxide films.
(b) Description of the Related Art
As semiconductor devices have been implemented for various applications, different device characteristics are required. For example, there can be a device such as a logic and central processing unit (CPU) merged with Dynamic Random Access Memory (DRAM) or Static Random Access Memory (SRAM).
In order to maintain the characteristics of different devices in one chip, it is required to fabricate the devices with the gate oxide films formed with different thicknesses. Even in a circuit device only integrated with identical devices such as DRAM or SRAM, it is desired to form the gate oxide films with different thicknesses so as to differentiate the operation voltages of the devices in one chip.
FIG. 1 shows a semiconductor device with 3 gate oxide films having different thicknesses fabricated according to a conventional method. Referring to FIG. 1, the method for forming the gate oxide films with different thicknesses will be described.
First, a gate oxide film 104 is formed on a high voltage region (A1), a middle voltage region (A2), and a low voltage region (A3) defined on a semiconductor substrate 100 by device isolation regions 102. Next, a photo resist film is formed on the first gate oxide film 104 and is patterned so as to expose the middle and low voltages regions A2 and A3. Then, the gate oxide film is removed on the middle and low voltages regions A2 and A3 using the photoresist as a mask so as to form the gate oxide film 104 only in the high voltage region A1.
Next, after removing the photoresist film, an annealing process is performed on the semiconductor substrate 100 so as to form a second gate oxide film. At this time, the second gate oxide film is formed thinner than the gate oxide film 104 for the high voltage region A1. Next, the second gate oxide film is patterned through photolithography so as to form a gate oxide film 106 only in the middle voltage region A2.
Finally, a gate oxide film 108 for the low voltage region is formed by oxidizing the semiconductor substrate 100, the gate oxide film 108 being formed thinner than the second gate oxide film.
However, the conventional method has a drawback in that the gate oxide films are formed through multiple etch processes such that the dielectric material of the device isolation regions 102, which has little etch selectivity with the gate oxide films 104, 106, and 108, is partially removed. Thus, the gate oxide films are damaged, causing the semiconductor devices to operate in a faulty manner, such as breaking down.